Method and apparatus for digitally measuring speed

Abstract

A digital display doppler radar unit has a moving mode and a stationary mode. The incoming doppler signal, which in the moving mode, represents both speed of the radar platform and speed of an approaching target vehicle, is separated into two signal components by selective filtering. One component represents the sum of the ground speeds for the radar platform and approaching vehicles. A time base is generated by a crystal control means and the time base is utilized for correlating the received doppler signals, indicative of speed, with the time base. Each doppler signal component is converted to binary coded decimal (BCD) information and a digital counter counts the cycles of each reeived doppler signal and compares the count with an amount previously stored. Circuit means are provided to validate the received doppler signal, allowing their continual processing only after a preselected number of valid comparisons are made. The radar signal component representing the ground speed sum for the radar platform and the approaching target vehicle, and the component representing radar platform speed are combined. The radar platform speed is subtracted from the combined component, resulting in a digital count representing approaching target vehicle speed. In the stationary mode, there is no radar platform doppler pulses in the radar return and consequently, the returning pulses may be processed directly and no subtracting function is performed. Speeds corresponding to the valid received doppler signals are suitably displayed to indicate a speeding violation.

Having thus described the invention, we claim: 
     
       1. A doppler radar device for use in law enforcement and service vehicle speed surveilance and for determining the speed of an approaching target vehicle when the radar platform vehicle is either moving or stationary, said device comprising: means for producing a continuously running time base;   a single means for receiving a composite doppler signal having components relating to the speed of said target vehicle and the speed of the radar platform vehicle;   means for separating said doppler frequency into a first frequency component and a second frequency component;   means for correlating the frequency of said first frequency component with said time base;   means for correlating the frequency of said second frequency component with said time base;   means for counting cycles of each of said time base correlated frequency components, said correlating means and said frequency component counting means cooperating to count each of said frequency components in a preselected time increment, said time increment relating to said time base;   means for subtracting said counted cycles of said first frequency component from said counted cycles of said second frequency component; and   means for displaying the result from said subtracting means in unit indicative of speed of said approaching vehicle.   
     
     
       2. The combination as in claim 1 wherein said means correlating said first and second frequency components with said time base includes gate means for initiating the counting of the cycles of said first frequency component after said first cycle of said second frequency component has passed. 
     
     
       3. The combination as in claim 2 wherein said initiating means includes a primary time base and a secondary time base counter, and means for initiating the counting of clock pulses corresponding to the occurrence of a preselected condition relative to said second frequency component and stopping the counting corresponding to a preselected condition relative to said first frequency component. 
     
     
       4. The combination as in claim 3 including means for generating control pulses, at least one of said control pulses being operable to restart the counting of said secondary time base counter. 
     
     
       5. The combination as in claim 1 wherein said device includes means for assuming a speed count of said platform vehicle,   means for storing said assumed speed count;   means for updating the speed count of said platform vehicle,   means for comparing said assumed speed count to said updated speed count; and   means for automatically changing said assumed speed count to said updated platform speed count at a rate of change not to exceed a predetermined maximum.   
     
     
       6. The combination as in claim 1 wherein said radar device includes a means for disabling said displaying means of said target means speed when a component of said received doppler signal could correspond to a closing rate between the platform vehicle and another vehicle rather than the platform vehicle speed relative to the surface. 
     
     
       7. The combination as in claim 6 wherein said disabling means includes means for storing an assumed speed count of said platform vehicle,   means for updating the speed count of said platform vehicle, means for comparing said assumed speed count to the updated speed count, and   means for automatically changing said assumed speed count to said updated platform speed count at a rate of change not to exceed a predetermined minimum, said disabling means being operable to preclude erroneous display of target vehicle speed during the operation thereof.   
     
     
       8. The combination as in claim 1 wherein said supporting means includes a filter network, said filter network including a high pass filter means for rejecting the platform vehicle speed frequency component. 
     
     
       9. The combination as in claim 1 wherein said separating means includes a filter network, said network having passing filter means for passing the platform vehicle speed frequency components and for rejecting the target vehicle speed components. 
     
     
       10. The combination as in claim 9 wherein said passing filter means is a band pass filter operable to reject erroneous third vehicle frequency components resulting from the closing speed of the platform vehicle and the said third vehicle. 
     
     
       11. The invention as in claim 10 including means for electronically controlling the passing frequency of said band pass and high pass filter means in response to an assumed speed count. 
     
     
       12. The invention as in claim 8 including means for electronically controlling the passing frequency of said band pass and high pass filter means in response to an assumed speed count. 
     
     
       13. The combination as in claim 1 wherein said separating means includes a filter network, said filter network including a high pass filter means for rejecting the platform vehicle speed frequency components and a passing filter means for passing the platform vehicle speed frequency component and rejecting the target vehicle speed components, and means for electronically controlling the passing frequency of said band pass and high pass filter means in response to an assumed speed count. 
     
     
       14. The combination as in claim 1 including automatic gain control means operable to preclude the exceeding of the dynamic range of said device prior to separating said doppler signal. 
     
     
       15. The invention as in claim 1 wherein said second frequency component corresponds to the difference in velocity between said moving platform vehicle and said approaching target vehicle and wherein said first frequency component corresponds to the velocity of said moving platform with respect to the surface. 
     
     
       16. A method for determining the speed of an approaching moving vehicle, said method comprising the steps of: receiving a composite doppler signal with a single receiving means;   separating said composite dopper signal into a first frequency component and a second frequency component;   digitally producing a precise time base;   digitally correlating said first doppler signal frequency component with said time base;   digitally correlating said second doppler signal frequency component with said time base;   counting the cycles of said first correlated doppler signal frequency component;   counting the cycles of said second correlated doppler signal frequency component;   subtracting the count of said first doppler signal frequency component from said second coppler signal frequency component; and   displaying the result of said subtracted count of correlated cycles.   
     
     
       17. The method as in claim 16 including the step of initiating the counting of the cycles of said first frequency component after said first cycle of said second component has passed. 
     
     
       18. The method as in claim 17 including the step of causing said displaying means to ignore said target vehicle speed when said received doppler signal could correspond to a  closing rate rather than target vehicle speed. 
     
     
       19. The method as in claim 16 including the step of causing said displaying means to ignore said target vehicle speed when said reached doppler signal could correspond to a closing rate rather than target vehicle speed. 
     
     
       20. The method as in claim 19 including the steps of storing an assumed speed count of said platform vehicle,   comparing said assumed speed count to an updated speed count, and   correcting said assumed speed count at a rate not to exceed a predetermined maximum.